This invention, in general, relates to rotary printing presses, for instance offset presses, and similar rotary presses. In a more specific aspect the invention pertains to lock-up assemblies for securing printing plates on the plate cylinders of such presses.
As explained in U.S. Pat. No. 4,688,483 there are two types of procedures for fastening printing plates on plate cylinders, also termed blanket cylinders or platen cylinders. One procedure entails securing the leading edge of the printing plate to the plate cylinder while the trailing edge lies freely on the cylindrical surface. In the second procedure the plate cylinder is provided with clamping units which grasp both the leading and trailing edges of the printing plate. This invention relates to this second procedure. Clamping mechanisms for effecting these procedures might also be categorized in two groups, those utilizing tensioning spindles--also known in the art as reel rods--and those utilizing other printing plate gripping and tightening means. Patents employing means other than tensioning spindles are exemplified by U.S. Pat. Nos. 1,937,119, 2,285,116, 4,688,473, 4,688,484 and 5,413,042.
It is clamping assemblies using tensioning spindles or reel rods with which this invention is concerned. Examples of these mechanisms are U.S. Pat. Nos. 3,757,690, 4,476,783, 4,951,568, 5,178,068, 5,315,931, 5,402,722, 5,461,981, and 5,503,072. In the utilization of offset presses in which the printing cylinder is provided with a tensioning spindle or reel rod, the first end of the printing plate usually has a hooked edge which is inserted in a holding gap in the printing cylinder. The printing plate is then wrapped around the printing cylinder and connected at a second end to a tensioning spindle, commonly known as a reel rod, disposed within the plate cylinder. The reel rod carries a slot for receiving the second end of the plate. After insertion of that plate end in this receiving slot, partial rotation of the reel rod tightens the printing plate. The tightening operation is quite critical because, as is generally known, the printing plate must be held on the printing cylinder with a certain amount of tension to ensure that it will stay in place and will not shift during the printing operation. Such shiffing can have a detrimental effect on the quality of the material printed by the press. Due to inaccurate tensioning some reel rods also can distort or tear the printing plate during the tensioning operation.
It is to be appreciated, then, that, as emphasized in U.S. Pat. No. 5,315,931, a substantial amount of skill on the part of the press operator is required to apply a printing plate to a plate cylinder. Frequently, one of the printing plate's edges can slip out of the holding gap in the printing cylinder before the other edge of the printing plate can be inserted into the receiving slot in the reel rod, where it is to be tensioned. This is particularly the situation when the cylinder is being rotated in order to wrap the printing plate around it. For this reason, in many instances at least two operators must attend to the placement of the printing plate on the printing cylinder. As pointed out in U.S. Pat. No. 4,476,783 one operator holds the printing plate, and the other engages the plate in its gap and slot while also trying to stretch the printing plate. In addition tools such as wrenches or drivers are frequently required. Another disadvantage of present lock-up assemblies is the difficulty of maintaining the gap in the printing cylinder in alignment with the slot in the reel rod while removing or installing the printing plate. In fact, the alignment process itself is not without problems. It can be seen, then, that there is a definite need for a press printing cylinder carrying a reel rod which overcomes these limitations. The clamping device herein is directed to that end.